Cholera is a global disease; endemic to Bangladesh, regions of South America, Africa and Australia, and also the Gulf Coast of the United States with the potential for epidemic in all aquatic environments. It is estimated that 120,000 people worldwide die from cholera annually. V. cholerae causes periodic, seasonal outbreaks in regions where it is an established member of the indigenous aquatic flora and this capacity is linked to its survival under diverse environmental conditions. V. cholerae switches its colonial morphology from smooth and translucent type to wrinkled and opaque type termed rugose variant when exposed to environmental stresses. We hypothesize that the phase variation mediated changes in population composition of V. cholerae can increase aquatic survival chances of the organism. The tong term goal of this project is to understand how V. cholerae survives between epidemics by focusing on the molecular mechanism of smooth to rugose phase variation, its physiological consequences and its effect on the aquatic survival of the organism. Towards this goal, we will focus on the following specific aims: 1) determine and characterize the molecular basis of the smooth to rugose phase variation, 2) characterize the transcriptional network governing rugose specific gene expression and characterize the physiological and behavioral changes in the organism resulting from phase variation 3) elucidate the effects of diverse environmental parameters on the aquatic survival properties of the smooth and rugose variants and on the frequency of phase variation. Understanding how the smooth to rugose phase variation is contributing to persistence and survival of V. choterae O1 El Tor in environmental aquatic habitats, and elucidation of the genes and processes regulating the phase variation will further our understanding of aquatic life cycle of an important human pathogen. Results obtained from this study should lead to the development of molecular tools that can be used to identify transcripts or proteins that are predicted to provide better environmental fitness to the organism in natural aquatic habitats. This information will prove useful in the prediction and/or control of cholera epidemics. Smooth to rugose phase variation presents another challenge in public heath since this process renders the organism resistant to oxidative stress and chlorine-mediated killing. Chlorination is used as a first line of defense against V. cholerae and many other waterborne pathogens. Understanding the mechanism and regulation of phase variation may aid in designing methods/inhibitors for modulating the frequency of phase variation and thus biocide resistance in V. cholerae and other aquatic pathogens.